The present invention relates to a bifocal optical pickup head, and more particularly to an optical pickup apparatus used with a single objective lens for accessing a compact disc (CD) and a digital versatile disc (DVD), which are different in thickness.
A technique of accessing data recorded on a surface of an optical disc such as CD or DVD by an optical pickup head has been known. In reading, a laser beam is focused on a data storage surface of the optical disc and the beam reflected from the optical disc is converted to an electric signal by a photo detector. In writing, the above procedures are inverted. The specifications of CD and DVD are shown in Table 1.
TABLE 1 Thick- NA (numerical ness (mm) Track pitch (.mu.m) Wavelength (nm) aperture) CD 1.2 1.6 780 0.45 DVD 0.6*2 0.74 635-650 0.6
As know from Table 1, in comparison with CD, DVD has a higher storage density. The differences in dimension (especially the storage density difference) result in various reading requirements for an optical pickup apparatus. To access data on a data storage surface of a CD or a DVD, an optical pickup head is a key component in the optical pickup apparatus, and optics is most critical in the optical pickup head.
Therefore, to access both CD and DVD by an optical pickup apparatus having a single objective lens, it is necessary to design an optics in which beam spot size is changed on the basis of two different numerical apertures.
To meet compatible requirement for CD and DVD, it is necessary to access both CD and DVD with a single optical pickup head, and how to obtain a bifocal optics is a critical technique.
Conventional techniques of accessing both CD and DVD with a bifocal optical pickup head are listed as follows:
1. Two objective lenses having different focal lengths corresponding to CD and DVD respectively are used, and a driving mechanism is utilized to select one of the objective lenses. Such a design has disadvantages of increased weight of an optical pickup head and increased manufacturing cost. PA0 2. A diffraction element is used to achieve bifocal effect. PA0 3. A holographic optical element is used for bifocus. However, it suffers from manufacturing difficulty and high manufacturing cost. PA0 4. A NA controller is constructed by a liquid crystal display (LCD) shutter such that an objective lens has two NA values, thereby achieving bifocus. However, the orientation of the polarization of a laser source needs to be accurately aligned, and the LCD should be continuously powered. PA0 5. An annular objective lens is used in which beams passing through center of the lens are for CD while beams passing though inner and outer rings thereof are for DVD. A similar technique is disclosed in U.S. Pat. No. 5,665,957 in which a hologram lens having a pattern thereon is used for bifocus. However, its manufacturing cost is high and mounting tolerance is low. PA0 6. Two light sources and two lens are used. For example, according to U.S. Pat. No. 5,777,970, two laser sources of different wavelengths correspond to respective lenses to provide respective focal lengths and spots of respective diameters. It has disadvantages of high manufacturing cost and complicated structure. PA0 7. A zooming aperture is utilized in association with a movable objective lens, as shown in U.S. Pat. No. 5,659,533 and U.S. Pat. No. 5,281,797.